1. Field of the Invention
This invention related to nuclear reactor fuel assemblies; and in particular, to such an assembly which includes a plurality of elongate fuel rods arranged relatively equi-distant and parallel in a matrix (bundle), this orientation being maintained by a plurality of improved spacer grid means disposed at prescribed support plane zones intermediate along the bundle length.
In fuel assemblies which comprise a matrix of parallel fuel rods, the ends of the rods and a number of intermediate points must be fixed in space using spacer means, such means acting to maintain relatively constant separation along the rod lengths. During reactor operation, especially with a reactor whose rods are subjected to passage of an operative fluid (e.g. water coolant), the fuel rods are subject to vigorous and harmful vibration from a number of sources; hence their spacers must comprise relatively strong, stiff members. "Fretting" and similar abrasion of a rod cladding by the tangent spacer contact can typically result from excessive vibration of the rod relative to its spacers. To avoid this, resilient spacer contact means are employed. Such resilient spacer contacts (springs) may be mounted from adjacent portions of spacing strips according to the invention. In a preferred embodiment, these strips are arranged in two orthogonal sets, sometimes known as an "egg crate" design. It will be apparent that this strip matrix may define rectangular spacing cells enclosing respective fuel rods, with contact springs projected therefrom to resiliently establish the rod position within the cell. Known spacer constructions do not adequately meet the requirements of todays reactors and are quite unsatisfactory in certain respects (see below). The present invention is adapted to meet this need.
2. Prior Art Problems; Background
Fuel bundles as presently known in the art do not satisfactorily answer certain operational problems. One such problem involves fuel rod vibration; for instance as induced by high velocity fluid flow (along the rod length). Fuel bundle design presents an especially severe vibration problem because of the multitude of closely-spaced elongate fuel rods stacked in parallel, with their thin protective sheathing (e.g. of Zircaloy). For proper reactor performance and to avoid destructive rod-contact, the rod position and relative spacing must be very closely controlled (kept constant) statically, and also dynamically, that is, during the high velocity coolant flow and the other extreme, rapidlychanging reactor conditions (such as the high operating temperatures, the corrosive environment and the like). Without adequate internal support the fundamental frequency of such a long fuel rod is very low (on the order of one cycle per second). The diverse pressure components from passing fluid impressed on a rod are likely to be quite influential at such low (response) frequencies and can induce the rod to vibrate wildly unless adequate support is provided. Furthermore, an unsupported rod is so extremely flexible (limp) that relatively small excitations may induce vibrations of great amplitude -- for instance, the excursions may be large enough to slap one rod against another, with, of course, catastrophic consequences for reactor operation. More importantly, even a small rod displacement can destroy the "critical geometry" condition which is so essential to proper safe reactor operation. The invention meets this problem by providing an improved safer rod spacer means and including contact springs (grid springs) whose spring characteristics are tailored to the rod stiffness and are set within a prescribed stiffness range: i.e. to be stiff enough to prevent relative motion of the rod, as well as to force a "near-node" on the rod under flow-induced vibration; yet to be resilient enough to accommodate rod dimensional changes such as axial expansion (as discussed below) without buckling the rod. Those skilled in this field will recognize that such a definition of grid spring characteristics is new in the art.
A second challenge is that a spacer support system must also be able to accommodate dimensional changes in the rods (axial and radial "growth" or shrinkage) such as may be caused by thermal expansion, creep, irradiation, pressure changes etc. as known in the arts. Thus, an adequate support system must accommodate such growth, while also providing support and stability under vibration. In particular, support springs must be able to follow rod motion (whether or not a "permanent set" is assumed) functioning so that any permanent set is small enough to preclude opening gaps beyond a prescribed maximum -- since this could introduce rod-to-spring gaps, leading to fretting problems associated with spring impact, etc. The invention also meets this problem by providing this improved support system with contact springs of sufficient "elastic springback" to "follow" all such dimensional changes.
A third problem involves buckling of fuel rods due to excessive axial loading, typically caused by friction forces. For instance, thermal- or irriadiation-induced elongation of a rod while it is gripped by its supports can compress the rods beyond their elastic limit whereupon they will buckle. Accordingly, an upper limit must be set on the support rod contact force so that the support (e.g., contact springs) will yield and allow rod axial motion before buckling pressure is attained. Moreover, such axial loading can also be imposed upon the support system itself; hence, the overall system must be specified for sufficient vertical strength to withstand axial thrusts on the grid. Also, with some (minor) vibrational displacement of a rod being inevitable, the overall support system must be designed to follow rod displacement however great, lest damaging impacts, fretting, etc., result. In other words, a grid spring must never "lose contact". The grid spring of the subject invention satisfies this requirement, being specified to exhibit elasticity over the maximum expected rod excursions induced by coolant-flow, vibration or other excitation such as in the course of handling and shipping.
Other problems in the support design may be contemplated by those in the art; such as minimizing hydraulic impedance (pressure drop) with anticipated coolant fluid flow and offering good neutron economy (low parasitic neutron absorption characteristics).
Accordingly and primarily to provide the mentioned improved spacer arrangement, a number of design features are taught herein being novel in themselves and/or in combination, and may be briefly summarized as follows in Table I.